The technology of vapor-liquid process towers is replete with various material designs used for tower packing. The type of tower packing is a function of the particular process to be affected. The packing elements may comprise a structured grid arranged to form a regular array inside the column or they may comprise oblique shapes dumped into and randomly arranged within the tower. The shape of the packing element thus determines the density of the stacked members, the resistance to flow caused thereby and defines the vapor-liquid interface characteristics. Prior art packing members have thus found utility in a variety of shapes, sizes and material forms in both structured array and dumped packing configurations.
Dumped tower packings have found application for considerable time in distillation processes, absorption and desorption processes, gas cleaning and related liquid-gas column process operations. Saddle shaped members have found utility as column packing elements in vapor-liquid contact towers of the kind used to bring about mass and energy transfer between liquid and vapor phases. The packing elements cause intimate flowing contact between the liquid and vapor, which contact is generally countercurrent. Vapor generally passes upwardly through the tower or column and liquid cascades downwardly through the packing. For this reason "packing density" is an important design aspect.
Varieties of prior art dump packings of the "saddle type" are set forth and shown in U.S. Pat. No. 4,303,599 issued in Dec. 1, 1981 to Ralph F. Strigle, Jr. et al and U.S. Pat. No. 4,333,892 issued on June 8, 1982 to Stephen R. M. Ellis et al. These packing elements are constructed of a generally elongated polygonal base member which is substantially flat transversely. The base member is curved about a notional axis to render the overall "saddle" shape. A bridging member is also provided which arcuately spans the curved base member. The bridge increases the strength of the member for permitting lighter weight material. Since the saddles are dumped in place, weight of the saddle stack is an important design aspect. Any bending or distortion of the packing elements affects the mass and heat-exchange efficiency of the process. Moreover, should an underlying packing element collapse, the flow area therethrough could be substantially diminished. In effect, the packing is designed to be sufficiently strong to resist bending and provided in a configuration that when dumped, the elements will not orient themselves out of randomness. Oriented layers of saddles would cause nesting and decrease the flow area to reduce efficiency. The bridge members of the prior art may be seen to limit, to some degree, such orienting of contiguous packing elements.
Another prior art packing element of the saddle variety is set forth in U.S. Pat. No. 4,256,673 issued on Mar. 17, 1981 and assigned to Raschig Gmbh. A double-curved bight portion packing element is shown therein to be constructed of plastic material. The side wall portions of the packing element are formed with relatively smooth corrugations forming a scalloped edge and a series of drip points. The corrugations stiffen the side walls as well as increasing the length of the flow path of the gas and liquid, which is an advantage to transfer efficiency. Such design features are particularly necessary with plastic packing elements due to the problems of structural strength and wetting.
Transversely extending corrugations improve such factors and serve to preclude unnecessary surface contact between parallel juxtaposed packing elements. When substantially solid surfaces, such as prior art saddle side walls come into contact in a tower, fluid flow is interrupted and efficiency reduced. For this reason ribs, holes and corrugations have been shown to improve the vapor-liquid contact and gas turbulence around the packing elements. Unfortunately, wetting is not uniform across a plastic surface and a small number of apertures have a limited effect on the flow pattern. It has thus been found necessary to use structured metal grid arrays, which provide the myriad of wetted vapor-liquid flow patterns and turbulence not achievable with conventional plastic saddles for certain process conditions.
The surface properties of the packing elements, whether in saddle configurations or otherwise, may thus be seen to play an important role in the efficacy of the contact between the liquid and the gas. Surface properties including the propensity of the material to wet and accept the flow of liquid thereacross have thus been emphasized. Materials, such as metal, clearly offer a greater wetting effect but are limited in construction and application.
Because of the practicalities of metal forming, it has not heretofore been feasible or common to form tower packing saddles from metal. This is particularly true of the extreme bidirectional curves of semi-toroidal configurations. The lateral expansion of sheet metal is quickly transformed into shear and/or latent stresses which if not immediately destructive, result in debilitating corrosion under normal tower environmental conditions. Instead, saddle elements have traditionally been constructed of ceramics, or more recently, from plastics as shown above. The weight of ceramic saddles has been shown to be a disadvantage due to structured problems and the breaking of saddles under their own weight. Plastic saddles, because of the properties of the material from which they are formed, are limited in the service temperature which they can endure in a tower environment. Moreover, both plastic and ceramic saddles will "wet" only with difficulty by many of the process liquids. Because of this operation factor the efficiency with which such saddles bring liquids into good exchange contact with vapors is impaired.
The advantage of material having good wetting properties is abundantly recognized in the prior art. However, the choice of material directly affects the possibilities of forming the element such as the saddle. The plastic saddles of the type set forth above are thus generally constructed by injection molding. Ceramic saddles are formed by extrusion and forming or by molding and subsequent firing. Metal saddles of the prior art have been made by punching, stamping and forming material, generally about a single notional axis. The material selection depends upon the expected conditions of heat, pressure and chemical reaction. U.S. Pat. No. 4,333,892 set forth above has taught the use of stamped, corrugated, dimpled and grooved material blanks for use in environments where wetting and heat are major considerations. A packing element formed about a single notional axis of a polygonal base member with a substantially flat transverse body portion is described therein. It is further suggested that such packing elements can be formed of solid or porous metal, metal gauze or expanded metal mesh. Because of the configuration of this and related prior art packing elements, strengthening members are necessarily utilized. Such strengthening members generally comprise the bridge span set forth above which connects intermediate body portions of the substantially flat arched wall regions formed about the single notional axis. The bridge member, of course, adds weight and cost to the design.
Certain other problems remain in prior art dumped packing designs. The bed produced from the dumped packing is constantly subject to vibration from the fluid flow therethrough. Vibration causes settling of non-interlocked members resulting in an increase in flow resistance. Moreover, the packing members which are not easily formed into a structurally sound configuration, affording the greatest process efficiency, are generally only producible from materials in which wetting is not facilitated. The use of metal has been limited to configurations wherein strength may be provided with a minimum of weight. Such shapes are often less efficient and more costly to produce than conventional designs. Moreover, the ability to provide a plurality of fluid transfer points on a packing element, wherein wetting is enhanced, may be severely limited by structural considerations. Although the "saddle variety" of dump packing has been shown to be efficient, such prior art disadvantages need to be overcome.
It would be an advantage therefore to provide a metal dump packing element in a saddle configuration affording ease in forming and overcoming the disadvantages the prior art saddle structures. The packing element of the present invention provides such a design wherein a saddle packing member is arcuately formed from expanded metal about orthogonal notional axis to form a segmented semi-toroidal body. In this manner, light weight, structurally sound packing elements having excellent wetting properties are provided in an interlocking configuation presenting minimum weight, a maximum of flow transfer points, and enhanced fluid flow-through characteristics.